Water treatment systems which remove oil and suspended solids from water to be treated containing oil and suspended solids by using filtration membranes are known. By configuring water treatment systems such that water to be treated is supplied from one end side of filtration membranes and discharged from the other end side of the filtration membranes, i.e., crossflow filtration is performed, it is possible to reduce clogging of the filtration membranes even in the case of treating water to be treated containing large amounts of oil and suspended solids, such as oilfield-produced water.
Furthermore, by mixing a gas with water to be treated, clogging of filtration membranes can be suppressed by the scrubbing effect of gas bubbles. As a method for mixing a gas with water to be treated, there has been proposed a method using an aspirator (also referred to as an ejector) which produces a negative pressure by means of the Venturi effect due to constriction of a passage of water to be treated and which aspirates a gas into water to be treated (refer to Japanese Unexamined Patent Application Publication No. 2009-148673).